Why underhook the leg in bottom side control?

So I was watching the little show Wanderlei put on before the Real Deal. During the video, Rafael Cordeiro gets him in side control. Wanderlei then purposely underhooks the near leg. I've never liked doing this, because you can get your arm trapped, and then kimura-d Matt Hughes style. Or in an MMA situation, you will just get hammerfisted with only one hand to attempt to defend yourself. I understand he can use it as leverage to throw him over, but first he'll need to get his weight off of him. What am I missing here? What is the big advantage?

I normally bring my elbow into their hip, and then go for the underhook on the otherside.

Possibly to stop the knees, or he may have just started that way so that he could push into the guy's arm pit with his other hand, to get the guy's weight off him (for example this is used as an x-guard entry). Maybe then he can create space to stand, or get his guard back.